This invention relates to monitoring the fuel flow out of a pulverizer and more particularly, to a method and apparatus for real time monitoring of the fuel flow rate through the outlet pipes of a pulverizer.
Each pulverizer supplying coal to a furnace for combustion, typically supplies pulverized coal to each burner front of a single elevation of burners. In this manner as the demand for steam increases, an additional pulverizer may be placed in service to provide pulverized coal to an additional elevation of burners. Similarly, as the demand for steam decreases an elevation of burners as well as the pulverizer providing pulverized coal thereto may be removed from service.
Typically, single furnaces such as tangentially fired pulverized coal furnaces firing air entrained pulverized coal, are rectangular in cross-section with four burners per elevation; each burner is typically located at a corner of the furnace. The coal delivery pipes terminating at each burner front of an elevation originate at a single pulverizer. No two coal delivery pipes from a pulverizer are the same length or traverse the same path. Thus, no two coal delivery pipes inherently have the same pressure drop. To compensate for the differing pressure drops in the coal delivery pipes, orifices are placed in the outlet pipes of each pulverizer to provide a uniform pressure drop in each coal delivery pipe between a pulverizer and the burner fronts to which that pulverizer supplies pulverized coal. The presumption is that with a uniform pressure drop in each coal delivery pipe there will be an even transport air flow distribution among the coal delivery pipes and further that a uniform fuel flow distribution among the fuel delivery pipes will follow the uniform transport air flow distribution.
It is possible to achieve an even transport air distribution, yet not achieve a uniform fuel flow distribution. A non-uniform fuel flow distribution to the coal delivery pipes results in more or less coal being delivered to one or more of the burners than is delivered to the other burners. In turn, the stoichiometry of combustion is effected such that the corner or corners receiving more coal is/are fuel rich while the remaining corner or corners is/are fuel lean. This results in uneven combustion causing an uneven temperature distribution. The cause of uneven fuel flow distribution is not well understood. Uneven fuel flow distribution is thought to be influenced by many factors including air flow patterns internal to the pulverizer and the differing, circuitous paths each coal delivery pipe takes between the pulverizer and a burner front.
What is needed is a method and apparatus for monitoring the fuel flow through each coal delivery pipe of a pulverizer. Such a fuel flow detection system would provide a real time indication of the fuel flow through each coal delivery pipe and could be used just to monitor the fuel flow through each coal delivery pipe, to provide data for sizing a fixed orifice for each coal delivery pipe or in a feedback control system could be used to vary the cross-section of an orifice structure having movable elements that form a variable diameter orifice.